1. Field of the Invention
The present invention relates to a uni-morph, bi-morph or other type of piezoelectric and/or electrostrictive film element which generates or detects displacement or force in the form of bending, deflection or flexure, and which can be used for actuators, filters, display devices, transformers, microphones, sounding bodies (such as loudspeakers), various resonators, oscillators, or vibrators, discriminators, gyros, sensors and other components and devices. The present invention is also concerned with a method for producing such piezoelectric or electrostrictive film elements. The term "element" used herein means an element which is capable of transducing or converting an electric energy into a mechanical energy, i.e., mechanical displacement, strain or vibrations, or converting a mechanical energy into an electric energy.
2. Discussion of the Related Art
In recent years, in the fields of optics and precision positioning or machining operations, there has been an increasing demand for an element whose displacement is controlled for adjusting or controlling an optical path length or a position of a member or component of a device, on the order of fractions of a micron (.mu.m), and a detecting element adapted to detect infinitesimal displacement of a subject as an electric change. To meet the demand, there have been developed piezoelectric and/or electrostrictive film elements (hereinafter referred to as "P/E film elements") used for actuators or sensors, which elements comprise a piezoelectric material such as a ferroelectric material, and utilize the reverse or converse piezoelectric effect to produce a mechanical displacement upon application of an electric field to the piezoelectric material, or utilize the piezoelectric effect to produce an electric field upon application of a pressure or mechanical stress. Among these elements, a conventional uni-morph type P/E film element has been favorably used for a loudspeaker, for example.
There have been proposed ceramic P/E film elements used for various purposes, as disclosed in JP-A-3-128681 (i.e., in the co-pending U.S. patent application Ser. Nos. 07/550,977, 07/860,128, 08/102,960, 08/384,469, 08/392,083 and 08/452,092) and in JP-A-5-49270 (i.e., in U.S. Pat. No. 5,210,455 and U.S. patent application Ser. No. 08/013,046), which were filed by the assignee of the present invention. One example of the disclosed elements has a ceramic substrate which has at least one window, and is formed integrally with a thin diaphragm which closes the window or windows so as to provide at least one thin-walled diaphragm portion or vibratile portion. On an outer surface of each diaphragm portion of the ceramic substrate, there is formed a piezoelectric/electrostrictive unit (hereinafter referred to as "P/E unit") which is an integral laminar structure consisting of a lower electrode, a piezoelectric/electrostrictive layer (hereinafter referred to as "P/E layer") and an upper electrode. The P/E unit is formed by a suitable film-forming method or process on the corresponding diaphragm portion of the ceramic substrate. The thus formed P/E film element is relatively small-sized and inexpensive, and can be used as an electromechanical transducer having high reliability. Further, this element has a high operating response, and provides a relatively large amount of displacement by application of a low voltage, with a relatively large magnitude of force being generated. Thus, the above-described element is favorably used as a member for an actuator, filter, display device, sensor or other components or devices.
To produce the P/E film element as described above, the lower electrode, P/E layer and upper electrode of each P/E unit are laminated in this order on the diaphragm portion of the ceramic substrate by a suitable film-forming method, and is subjected to heat treatment (firing or sintering) as needed, so that the P/E unit is formed integrally on the diaphragm portion. A further study of the inventors of the present invention revealed that the piezoelectric/electrostrictive characteristics of the element are deteriorated due to the heat treatment (firing or sintering) effected during the formation of the P/E unit, more specifically, the P/E layer.
Described in detail, the P/E layer suffers from stresses due to firing shrinkage of the P/E layer or P/E unit which is in contact with the diaphragm portion of the ceramic substrate, during the heat treatment of the P/E layer. The stresses remain therein after the firing, and prevent the P/E film element from having a sufficiently dense sintered structure. In this case, the P/E film element can not exhibit desired or intended piezoelectric/electrostrictive characteristics. The residual stresses after the firing of the P/E layer undesirably deteriorate the piezoelectric/electrostrictive characteristics, in particular, reduces an amount of displacement of the diaphragm portion generated upon actuation of the P/E unit.
To produce an actuator for a display device, for example, a plurality of windows are formed in a suitable pattern through a ceramic substrate, and P/E units as described above are formed on respective thin-walled diaphragm portions that close the corresponding windows. In this case, the amounts of displacement of the diaphragm portions may be considerably reduced due to the stresses remaining in the P/E layer after the firing thereof, particularly when two or more adjacent P/E units are actuated at the same time, as compared with the displacement amount where a single P/E unit is actuated. That is, when the two adjacent P/E units are actuated simultaneously, for example, the displacement of the diaphragm portion which carries one of the two P/E units interferes with that of the diaphragm portion which carries the other P/E unit, resulting in reduction of the amounts of displacement of these diaphragm portions. The reduction in the amounts of displacement of the diaphragm portions is serious where the spacing between the adjacent P/E units is reduced to meet a recent demand for a larger number of the P/E units per unit area.